Move some declarations to mdebugread.h

This moves a couple of mdebugread-related declarations from symfile.h
to mdebugread.h, which seemed more appropriate.

gdb/ChangeLog
2019-01-10  Tom Tromey  <tom@tromey.com>

	* symfile.h (mdebug_build_psymtabs, elfmdebug_build_psymtabs):
	Don't declare.
	* mipsread.c: Include mdebugread.h.
	* mdebugread.h (mdebug_build_psymtabs, elfmdebug_build_psymtabs):
	Declare.
	* elfread.c: Include mdebugread.h.